1. Field of the Invention
This invention relates generally to magnetic transducers for reading information signals from a magnetic medium and, in particular, to a novel structure for a giant magnetoresistance sensor suitable for ultra high density data applications and to magnetic recording systems which incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni--Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the GMR effect (also referred to as the SV effect). In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe--Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In the SV sensor, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated from each other by a central region 102. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed by an antiferromagnetic (AFM) layer 125. Free layer 110, spacer 115, pinned layer 120 and the AFM layer 125 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current I.sub.s from a current source 160 to the MR sensor 100. Sensing means (a detector) 170 connected to leads 140 and 145 senses (detects) the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk).
Another type of SV sensor currently under development is an antiparallel (AP)-pinned SV sensor. In AP-Pinned SV sensors, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The AP-Pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG. 1. This improved exchange coupling increases the stability of the AP-Pinned SV sensor at high temperatures which allows the use of corrosion resistant antiferromagnetic materials such as NiO for the AFM layer.
FIG. 2 shows a prior art AP-Pinned SV sensor 200 comprising end regions 204 and 206 separated from each other by a central region 202. A free layer 210 is separated from a laminated AP-pinned layer structure 220 by a nonmagnetic, electrically-conducting spacer layer 215. The magnetization of the laminated AP-pinned layer structure 220 is fixed by an AFM layer 230. The laminated AP-pinned layer structure 220 comprises a first ferromagnetic layer 222 and a second ferromagnetic layer 226 separated by an antiparallel coupling (APC) layer 224 of nonmagnetic material. The two ferromagnetic layers 222, 226 (PF1 and PF2) in the laminated AP-pinned layer structure 220 have their magnetization directions oriented antiparallel, as indicated by the arrows 223, 227 (arrows pointing into and out of the plane of the paper respectively). The AFM layer 230 is formed on a seed layer 240 deposited on the substrate 250. To complete the central region 202 of the SV sensor, a capping layer 205 is formed on the free layer 210. Hard bias layers 252 and 254 formed in the end regions 204 and 206, respectively, provide longitudinal bias for the free layer 210. Leads 260, 265 provide electrical connections for the flow of the sensing current I.sub.s from a current source 270 to the SV sensor 200. Sensing means 280 connected to leads 260, 265 senses the change in the resistance due to changes induced in the free layer 210 by the external magnetic field (e.g., field generated by a data bit stored on a disk).
Prior art AP-Pinned SV sensors use an AFM in order to pin the pinned layer magnetization, however, each AFM has a blocking temperature at which the pinning field becomes zero. If the SV sensor temperature approaches the blocking temperature, the pinned layer magnetization can change its orientation resulting in degraded SV sensor performance.
Most commonly used antiferromagnetic materials have blocking temperatures (temperature at which the pinning field reaches zero Oe) near 200 C. This means that if the temperature of the SV sensor approaches this temperature, the pinned layer magnetization can change its orientation resulting in degraded SV sensor performance.
As magnetic data storage densities increase above the 10 Gbit/IN.sup.2 level, the required linear bit density becomes sufficiently high that the thickness of the active layers of the SV sensor become a limiting factor in magnetic storage file system capacity. In SV sensors using antiferromagnetic material pinning layers, the thickest component of the active layers is the antiferromagnetic layer. Therefore, significant reduction in SV sensor thickness can only be achieved by significantly reducing the antiferromagnetic layer thickness.
Therefore there is a need for a magnetoresistive sensor that eliminates the temperature limitations imposed by the blocking temperature characteristics of the commonly used antiferromagnetic materials required in prior art SV sensors for providing pinning fields. There is also a need for a magnetoresistive sensor having a significantly reduced thickness in order to meet the requirements of high areal density data.